I’ve received considerable response to yesterday’s post, "Breathalyzer Inaccuracy: Testing During the Absorptive State", including questions concerning the accuracy of breath machines after the absorptive state. Even in the post-absorptive state — that is, when the body has reached a state of equilibrium, or uniform distribution of alcohol — there are numerous sources of error attributable entirely to physiological factors. Simpson’s research has found that breath tests are inherently unreliable, indicating uncertainty levels of 15 to 27 percent. In an article written shortly before the one cited yesterday, he noted:
Over 90% of this uncertainty is due to biological variables of the subject, and at least 23% of subjects will have their actual blood alcohol concentration overestimated. Manufacturers’ specifications for the accuracy and precision of these instruments are inconsistent with the experimental values reported in the literature and I recommend that an appropriate amount of uncertainty be reflected in the results from these breath analyzers, especially when they are used for law-enforcement purposes.
Simpson, "Accuracy and Precision of Breath Alcohol Measurements for Subjects in the Absorptive State", 33(2) Clinical Chemistry 261 (1987). Another noted expert, Professor Michael Hlastala, Professor of Physiology, Biophysics and Medicine at the University of Washington’s Medical School, concludes:
Breath testing, as currently used, is a very inaccurate method for measuring BAC. Even if the breath testing instrument is working perfectly, physiological variables prevent any reasonable accuracy…
Hlastala, "Physiological Errors Associated with Alcohol Breath Testing", 9(6) The Champion 19 (1985).